Disk drive including means for preventing rotation

ABSTRACT

A motor controller includes DC motor, driving mechanism, voltage generator circuit and control unit. The driving mechanism moves an object having a predetermined mass against a load applied to the object and by transmitting a rotational force of the motor to the object. The voltage generator circuit generates first and second voltages. The first voltage is high enough to rotate the motor to such a degree as to get the object moved by the driving mechanism. The second voltage has the same polarity as the first voltage and has such amplitude as to prevent the motor from rotating either in a backward direction due to a cogging torque or in a forward direction. The control unit controls the voltage generator circuit in such a manner that the voltage generator circuit applies the second voltage to the motor after having applied the first voltage to the motor.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of pending U.S. patentapplication Ser. No. 10/195,086, filed Jul. 12, 2002. The enumeratedprior application is hereby incorporated in its entirety.

FIELD OF THE INVENTION

[0002] The present invention generally relates to a motor controller andmore particularly relates to an apparatus for performing an accuratepositioning control by using a DC motor of a small size and a method ofdriving a DC motor.

DESCRIPTION OF THE RELATED ART

[0003] A DC motor including a brush and a commutator has a simplestructure and can be manufactured at a low cost. Also, a DC motorachieves high efficiency and high output although its size is small, andneeds no special driver. For these reasons, a DC motor is now used innumerous appliances.

[0004] However, if it is necessary to control the stop angle of therotor of a DC motor accurately enough or to rotate the motor at anextremely low number of revolutions per minute, it might be difficultfor a DC motor to satisfy these requirements fully.

[0005] This is partly because a DC motor generates a cogging torque.Hereinafter, the cogging torque of a DC motor will be described.

[0006] As shown in FIG. 8A, a DC motor includes a rotor 94 and fields95. The rotor 94 includes magnetic poles 91, 92 and 93, each of whichincludes an iron core made of a magnetic material such as silicon steeland a coil that is wound around the iron core. Each of the fields 95 isa permanent magnet such as a ferrite magnet. The DC motor typically hasthree magnetic poles 91, 92 and 93 and two fields 95 as shown in FIG.8A.

[0007] In a DC motor like this, the magnetic poles 91, 92 and 93,including magnetic bodies, are attracted to the fields 95. Accordingly,even when no electrical power is applied to the DC motor, a torque isgenerated in such a direction as to rotate the rotor 94. The torquerotates the rotor 94 so that the magnetic poles 91, 92 and 93 arestabilized in the magnetic field that has been generated by the fields95. The torque that is going to rotate the rotor 94 is generated by theattraction between the magnetic poles 91, 92 and 93 and the fields 95.Thus, the angle of rotation of the rotor 94 at which the magnetic poles91, 92 and 93 are stabilized changes with the positional relationshipbetween the fields 95 and the magnetic poles 91, 92 and 93.

[0008] The “stabilized state” normally refers to a state in which one ofthe magnetic poles 91, 92 and 93 is closest to one of the fields 95. Forexample, FIG. 8A illustrates one of those stabilized states in which themagnetic pole 91 is closest to the N pole field 95. In such a state,there is no torque that is going to rotate the rotor 94.

[0009] Suppose the rotor 94 is rotated clockwise from the position shownin FIG. 8A. In that case, when the rotor 94 is rotated 60 degrees fromthe position shown in FIG. 8A, the magnetic pole 92 will be closest tothe S pole field 95 as shown in FIG. 8B. This is another stabilizedstate. Since the rotor 94 has the three magnetic poles 91, 92 and 93 andthe number of the fields 95 is two, there will be six stabilized statesfor one rotation of the rotor 94. That is to say, every time the rotor94 rotates 60 degrees, one of the six stabilized states appears.

[0010]FIG. 9 shows the magnitudes and directions of the torques that aregenerated by the magnetic attraction between the rotor 94 and the fields95. In FIG. 9, the state shown in FIG. 8A is regarded as an initialstate. If a force is externally applied to the rotor 94 to rotate therotor 94 clockwise from the initial state as indicated by the point A inFIG. 9 with no electrical power applied to the DC motor, the attractionbetween the N pole field 95 and the magnetic pole 91 generates a torquein the direction opposite to the rotational direction. As the angle ofrotation increases, this reverse torque increases its magnitude. Andwhen the rotor 94 rotates approximately 15 degrees, the magnitude of thereverse torque is maximized as indicated by the point B in FIG. 9. Asthe rotor 94 is further rotated, attraction is soon generated betweenthe magnetic pole 92 and the S pole field 95. Accordingly, the reversetorque applied to the rotor 94 decreases gradually. And when the rotor94 rotates approximately 30 degrees, no reverse torque is applied to therotor 94 anymore as indicated by the point C in FIG. 9.

[0011] As the rotor 94 is further rotated, the attraction between themagnetic pole 92 and the S pole field 95 dominates, thereby generating atorque that rotates the rotor 94 clockwise. When the rotor 94 rotatesapproximately 45 degrees, the torque that rotates the rotor 94 clockwiseis maximized as indicated by the point D in FIG. 9. But that torque alsodecreases as the rotor 94 is further rotated. And when the rotor 94rotates approximately 60 degrees, no torque that rotates the rotor 94clockwise is applied to the rotor 94 anymore as indicated by the point Ein FIG. 9.

[0012] Actually, though, a friction torque is applied to the shaft ofthe rotor 94 as indicated by the one-dot chains in FIG. 9. Accordingly,unless a torque that has a magnitude greater than that of the frictiontorque is generated and applied to the rotor 94, the rotor 94 neverrotates. Thus, the effective torque applied to the rotor 94 is indicatedby the solid curve in FIG. 9. As can be seen from FIG. 9, such a torquevariation is repeatedly caused every time the rotor 94 rotates 60degrees. As indicated by the solid curve in FIG. 9, the effective torquemay be positive, negative or zero depending on the angle of rotation ofthe rotor 94. This effective torque is the so-called “cogging torque”.

[0013] If the DC motor is stopped by discontinuing the supply of powerto the DC motor, the magnitude and the direction of the cogging torquechange with the stop angle of the rotor 94. Accordingly, when the rotor94 reaches such an angle as to generate zero cogging torque (e.g.,approximately 30 degrees or approximately 60 degrees in the exampleshown in FIG. 9), the rotor 94 can be stopped without being affected bythe cogging torque.

[0014] However, if the rotor 94 should be stopped at such an angle as togenerate a positive cogging torque, then that cogging torque is appliedto the rotor 94, thereby rotating the rotor 94 excessively (i.e., to anangle greater than the desired angle) until the rotor 94 is stabilized.In the example shown in FIG. 9, the rotor 94 is rotated unintentionallyto around 60 degrees, around 120 degrees, etc. On the other hand, if therotor 94 should be stopped at such an angle as to generate a negativecogging torque, then a cogging torque is generated and applied to therotor 94 in such a direction as to rotate the rotor 94 in the backwarddirection. In that case, just before the rotor 94 stops rotating, therotor 94 rotates in the backward direction until the rotor 94 isstabilized. In the example shown in FIG. 9, the rotor 94 retrogrades toaround 0 degrees, around 60 degrees, etc. For these reasons, when a DCmotor is used, it is difficult to control the stop angle of the rotor 94accurately enough.

[0015] Furthermore, a non-uniform torque is generated around the shaftof a DC motor when power is supplied to the DC motor. A DC motor of asmall size, in particular, has a small number of magnetic poles, andtherefore, there is a significant variation in the torque generatedduring one rotation of the rotor 94. The output of a DC motor is alsoaffected by a variation in the power supplied to the DC motor to driveit or in the load connected to the motor. Consequently, there is a greatvariation in the output of the DC motor.

[0016] In addition, there is also a great variation in the load appliedto a DC motor (e.g., friction caused at the bearing thereof). Inparticular, a load variation resulting from the difference betweenstatic friction and kinetic friction is a problem. More specifically,when power is supplied to a DC motor, the static friction caused at thebearing increases proportionally to the torque generated at the rotor94. However, once the rotor 94 has started to rotate, the staticfriction changes into kinetic friction. Accordingly, the friction thatinterferes with the rotation of the rotor 94 decreases steeply. Such avariation in friction may be regarded as a sort of negative resistance.Thus, in performing a proportional control on a DC motor, such avariation introduces instability into the system. Consequently, it isparticularly difficult to rotate the motor stably at a low velocity.

[0017] The rotor has a great moment of inertia, which poses anotherserious problem. In a DC motor of a small size, permanent magnets areused as its fields to cut down the space for the fields, and a rotorhaving a relatively large diameter may be used by making use of theextra space. In this manner, a high-efficiency and high-output motor isachievable. However, the larger the diameter of the rotor, the greatermoment of inertia the rotor has. The equivalent mass of the moment ofinertia of a rotor is changeable with the type of the load to be drivenby a DC motor, but typically several times as great as the mass of theload to be driven by the DC motor.

[0018] As the moment of inertia of a rotor increases, it takes a longerand longer time to start or stop a DC motor. Accordingly, the load beingdriven by the DC motor cannot move so quickly for a while after the DCmotor has been started. Likewise, it is also difficult for the DC motorto stop the load the instant the power that has been supplied to the DCmotor is stopped.

[0019] These problems may be solved if the velocity of the load beingdriven by the DC motor is detected to position the load accurately.However, when such a control mechanism is added to detect the velocityof the load, the cost of a DC motor controller increases. Accordingly, avelocity detecting mechanism like that cannot be added to an apparatusthat should be manufactured at a low cost.

[0020] Examples of controllers using a DC motor include an optical diskdrive. For example, Japanese Laid-Open Publication No. 2000-20974discloses a technique of pulse-driving a DC motor in an optical diskdrive. An optical disk drive needs to move an optical head (i.e., theobject of control) to a target location at a high speed and position thehead accurately. Hereinafter, the conventional optical disk drivedisclosed in Japanese Laid-Open Publication No. 2000-20974 will bedescribed.

[0021]FIG. 10A is a block diagram illustrating the main section of anoptical disk drive 101 that uses the conventional motor controller,while FIG. 10B is a plan view thereof. The optical disk drive 101 is aCD-ROM drive for use to read an optical disk (i.e., a CD-ROM in thiscase) 102, on which spiral tracks are formed. The optical disk drive 101includes optical head (optical pickup) 103, optical head movingmechanism (see FIG. 10B), control unit 109, tracking error signalgenerator 121, tracking servo circuit 122, sled servo circuit 123 andcomparator 124. The optical head 103 can be moved by the optical headmoving mechanism in the radial direction of the optical disk 102 mounted(i.e., the direction indicated by the arrow A in FIG. 10B). The radialdirection of the optical disk 102 will be herein simply referred to as a“radial direction”.

[0022] The optical head 103 includes an objective lens (or condenserlens) 132 and a tracking actuator 141. The objective lens 132 is movableboth in the radial direction and in a direction parallel to the axis ofrotation of the optical disk 102 (which direction will be herein simplyreferred to as a “rotation axis direction”). The tracking actuator 141moves the objective lens 132 in the radial direction (i.e., toward theinner periphery or the outer periphery of the optical disk 102). When apredetermined voltage is applied to the tracking actuator 141 by way ofa driver 142, the tracking actuator 141 moves the objective lens 132 inthe radial direction in accordance with the polarity and amplitude ofthe voltage.

[0023] The optical head moving mechanism includes sled motor (or feedmotor) 107, driver 171 for driving the sled motor 107, lead screw (wormgear) 181 secured to the shaft 108 of the sled motor 107, worm wheel241, pinion gear 242, rack gear 115 and a pair of guide shafts 116 forguiding the optical head 103 thereon. The optical head 103 is supportedby the guide shafts 116 so as to be movable on the shafts 116. When thesled motor 107 is started by a driving control technique to be describedlater, the optical head 103 starts to move on the guide shafts 116 in apredetermined direction.

[0024] The control unit 109 is normally a microprocessor (or CPU) toperform an overall control on the optical head 103, sled motor 107,tracking servo circuit 122, sled servo circuit 123 and other componentsof the optical disk drive 101. This control unit 109 includes a pulsegenerator (or pulse voltage generator) 191. The control unit 109 and thecomparator 124 together make up a shift detector for detecting the shiftof the objective lens 132.

[0025] In this optical disk drive 101, the output voltage signal of theoptical head 103 is input to the tracking error signal generator 121,which generates a tracking error signal TE as a voltage signal. Thetracking error signal TE is input to the tracking servo circuit 122,thereby generating a tracking servo signal TS as another voltage signal.The level (or the voltage value) of this tracking servo signal TSrepresents the magnitude and direction of the shift of the objectivelens 132 from its home position in the radial direction.

[0026] The tracking servo signal TS is input not only to the trackingactuator 141 by way of the driver 142 but also to the sled servo circuit123. In response to the tracking servo signal TS, the tracking actuator141 is driven so as to move the objective lens 132 toward the center ofthe target track. That is to say, a tracking servo control is performed.

[0027] However, it is still difficult for the objective lens 132 tofollow the track accurately just by driving this tracking actuator 141.For that reason, the sled motor 107 is also driven to move the opticalhead 103 itself in the direction in which the objective lens 132 hasmoved. In this manner, a sled control is carried out so as to move theobjective lens 132 back to its home position.

[0028] In response to the tracking servo signal TS, the sled servocircuit 123 generates a sled servo signal SS. The level (or the voltagevalue) of this sled servo signal SS represents the magnitude anddirection of the shift of the objective lens 132 from its home positionin the radial direction. The sled servo signal SS is input to thecomparator 124, which digitizes the signal SS. Then, the output digitalsignal (or voltage) of the comparator 124 is input to the control unit109.

[0029]FIGS. 11A, 11B and 11C are timing diagrams showing the respectivewaveforms of the sled servo signal (or voltage) SS, the output signal(or voltage) of the comparator 124 and the output signal (or voltage) ofthe pulse generator 191 in the optical disk drive 101 that uses theconventional motor controller. FIG. 12 is a flowchart showing how thecontrol unit 109 performs the sled control operation.

[0030] As shown in FIGS. 11A and 11B, if the sled servo signal SS has alevel (or voltage value) equal to or higher than a threshold level(i.e., a reference voltage value), the output signal of the comparator124 is high (H). On the other hand, if the sled servo signal SS has alevel (or voltage value) lower than the threshold level (i.e., thereference voltage value), the output signal of the comparator 124 is low(L).

[0031] In the optical disk drive 101, when the output signal of thecomparator 124 rises from the L level to the H level, the shift of theobjective lens 132 from its home position is regarded as having reacheda certain limit. Then, the pulse generator 191 of the control unit 109generates and outputs a pulse signal (or pulse voltage) having apredetermined pattern.

[0032] As shown in FIG. 11C, the pattern of the pulse voltage generatedby the pulse generator 191 is predefined in such a manner that when thesled motor 107 is driven responsive to the pulse voltage, the motor 107moves the objective lens 132 back to its home position. The output pulsevoltage of the pulse generator 191 is applied to the sled motor 107 byway of the driver 171. In response to the pulse voltage, the sled motor107 is driven, thereby moving the optical head 103 in the direction inwhich the objective lens 132 has moved and getting the objective lens132 back to its home position.

[0033] As also shown in FIG. 11C, each pulse voltage consists of firstand second (positive and negative) pulse voltages 151 and 152 havingmutually different polarities. The absolute value of the first pulsevoltage 151 is sufficiently greater than that of the start voltage ofthe sled motor 107. As used herein, the “start voltage” of the sledmotor 107 is a minimum voltage that needs to be applied to the sledmotor 10 to start it in the optical disk drive 101. In this case, theabsolute value of the first pulse voltage 151 may be about 120% to about170% of that of the start voltage of the sled motor 107. On the otherhand, the absolute value of the second pulse voltage 152 is smaller thanthat of the first pulse voltage 151. In this case, the absolute value ofthe second pulse voltage 152 may be about 50% to about 90% of that ofthe first pulse voltage 151.

[0034] When these pulse voltages are applied to the sled motor 107 byway of the driver 171, the sled motor 107 starts, and then accelerates,its rotation responsive to the first pulse voltage 151 but is braked andstopped responsive to the second pulse voltage 152 so as to get theobjective lens 132 back to its home position.

[0035] Next, it will be described with reference to FIG. 12 how thecontrol unit 109 performs its control over the sled motor 107. First, inStep 201 shown in FIG. 12, the control unit 109 determines whether ornot the output signal (or voltage level) of the comparator 124 haschanged from L level into H level. If the answer to the query of Step201 is NO, then the pulse generator 191 outputs no pulse signal (orpulse voltage) in Step 202. In that case, the processing returns to Step201 to start the processing steps all over again.

[0036] On the other hand, if the answer to the query of Step 201 is YES,then the pulse generator 191 generates and outputs the pulse signal (orpulse voltage) in Step 203 as described above. Thereafter, theprocessing returns to Step 201 to start the processing steps all overagain. In this manner, even if any variation has been caused in the loadof the optical head moving mechanism, the sled motor 107 still can berotated and driven stably and accurately.

[0037] Although the rotor of the DC motor has a great moment of inertia,the conventional optical disk drive described above can stop the rotorquickly, thus increasing the accuracy of control to a certain degree.The conventional optical disk drive, however, has the followingdrawbacks.

[0038] Firstly, it is difficult for the conventional optical disk driveto control the rotation of the DC motor at a sufficiently small stepangle. Thus, the conventional optical disk drive cannot achieve desiredhigh positioning accuracy. To achieve a high resolution by minimizingthe step angle, the pulse voltage applied to the DC motor to drive itmay have its amplitude (i.e., the level) or pulse width reduced.However, if the level of the pulse voltage is decreased, then the DCmotor will be affected by the variation in load or torque more easily,thus making it difficult to minimize the step angle.

[0039] On the other hand, if the pulse width is shortened, then themotor might just vibrate but not rotate at all at a certain angle ofrotation or less. This phenomenon is brought about by the coggingtorque. Specifically, even if the rotor of the motor has rotated to justa small degree by applying the pulse voltage with such a short widththereto, the cogging torque, which rotates the rotor to the oppositedirection, is generated and applied to the rotor the instant the drivevoltage reaches zero. Thus, the rotor returns to its original position.Accordingly, no matter how many times the same short pulse is appliedrepeatedly, the rotor will not rotate.

[0040] This phenomenon becomes even more noticeable if the drivingmechanism that connects the motor to the object of control gets looseand unfixed or exhibits some elasticity due to the deformation. In thatcase, the motor just vibrates and cannot move the object of control atall. Accordingly, once such a phenomenon has occurred, it is virtuallyimpossible to minimize the step angle by decreasing the amplitude of thepulse voltage or by shortening the pulse width thereof. Thus, theconventional control technique cannot increase the positioning accuracyexcept a very limited situation.

[0041] Furthermore, if the braking pulse is applied to the motor justafter the driving pulse has been applied thereto, then the stability ofcontrol might be decreased significantly. Also, the load of a drivingmechanism is not always the same both in the forward and backwarddirections. Rather, when the step angle is that small, the load in theforward direction is often different from the load in the backwarddirection. A difference like that is particularly remarkable when thedriving mechanism that connects the motor to the object of control haslow rigidity.

[0042] For example, some motor controller might get the object ofcontrol pressed by a compressed spring so to speak if the controllercontinues to drive the motor in the same direction. In that case, theload is very heavy for the motor in the forward direction (i.e., thedirection in which the motor should move the object) but is extremelysmall in the backward direction. If a braking pulse, which is smaller inamplitude than the driving pulse by a predetermined percentage, isapplied to the motor in such a state, then the motor will rotate in thebackward direction. In the worst-case scenario, even the object ofcontrol also moves backward, thus deteriorating the stability of controlconsiderably. Accordingly, the braking pulse should have its voltagevalue and pulse width defined in such a manner as to eliminate thatphenomenon. Thus, it is rather imaginable that a braking pulse havingsuch limited voltage value and pulse width is effective insufficientlyagainst the great moment of inertia of the rotor.

[0043] In the conventional control technique described above, theamplitude of the pulse voltage applied to the DC motor is about 1.7times as high as that of the minimum voltage that needs to be applied tothe DC motor to start it, thereby attempting to decrease the step angleas much as possible. Also, if the object has moved to less than thepredetermined magnitude responsive to the pulse voltage that has beenapplied to the motor a single time, the same pulse voltage is supposedto be applied a number of times.

[0044] However, if the predetermined magnitude of movement is notachieved upon the single application of a pulse voltage having such asmall level, then it is highly probable that the object will not move,either, no matter how many times the same small pulse voltage is appliedto the motor. The reason is as follows. For example, in theabove-described situation where the object of control is getting pressedby a compressed spring so to speak, the slight movement of the object ofcontrol further compresses the spring. Thus, in that case, a heavierload will be constituted by the object of control once the pulse voltagehas been applied to the motor.

SUMMARY OF THE INVENTION

[0045] In order to overcome the problems described above, an object ofthe present invention is to provide a motor controller that can performa stabilized and accurate positioning control using a DC motor and amethod of driving a DC motor.

[0046] Another object of the present invention is to provide a diskdrive including such a motor controller.

[0047] A motor controller according to a preferred embodiment of thepresent invention includes a DC motor, a driving mechanism, voltagegenerating means and control means. The driving mechanism is provided tomove an object having a predetermined mass against a load applied to theobject and by transmitting a rotational force of the DC motor to theobject. The voltage generating means generates first and secondvoltages. The first voltage has amplitude high enough to rotate the DCmotor to such a degree as to get the object moved by the drivingmechanism, while the second voltage has the same polarity as the firstvoltage and has such amplitude as to prevent the DC motor from rotatingeither in a backward direction due to a cogging torque or in a forwarddirection. The control means controls the voltage generating means insuch a manner that the voltage generating means applies the secondvoltage to the DC motor after having applied the first voltage to the DCmotor.

[0048] In one preferred embodiment of the present invention, the controlmeans preferably gets a pulse voltage applied as the first voltage tothe DC motor. A pulse width T of the pulse voltage preferably satisfiest≦T≦5 t, where t is an electrical time constant of the DC motor.

[0049] In another preferred embodiment of the present invention, thefirst voltage is preferably three times or more as high as a minimumvoltage that needs to be applied to the DC motor for the drivingmechanism to overcome the load and move the object.

[0050] In this particular preferred embodiment, the control meanspreferably senses the magnitude of movement of the object. If thecontrol means senses that the object has moved to less than apredetermined value, the control means preferably gets the first andsecond voltages repeatedly applied from the voltage generating means tothe DC motor.

[0051] More particularly, the control means preferably senses themagnitude of movement of the object. If the control means senses thatthe object has moved to less than the predetermined value, the controlmeans preferably gets the pulse width T of the pulse voltage increased.

[0052] In still another preferred embodiment, the voltage generatingmeans preferably generates the first and second voltages that are bothpositive or both negative. The control means preferably controls thevoltage generating means in such a manner that the voltage generatingmeans selectively applies the positive first and second voltages or thenegative first and second voltages to the DC motor. The first and secondvoltages preferably change their polarity from positive into negative,or vice versa, depending on a direction in which the object should bemoved.

[0053] In yet another preferred embodiment, the control means preferablycontrols the voltage generating means in such a manner that the voltagegenerating means alternately applies the first and second voltages tothe DC motor.

[0054] In yet another preferred embodiment, the voltage generating meansmay include a pulse width modulator and may generate the first andsecond voltages as effective pulse voltages to be output from the pulsewidth modulator.

[0055] In yet another preferred embodiment, if the object is not drivenfor a predetermined amount of time or more, the amplitude of the secondvoltage may be decreased.

[0056] In yet another preferred embodiment, the voltage generating meanspreferably includes switching means and generates the first voltage byturning the switching means ON.

[0057] A disk drive according to another preferred embodiment of thepresent invention includes driving means, a head, a head movingmechanism, a DC motor, voltage generating means and control means. Thedriving means rotates a disk. The head reads and/or writes informationfrom/on the disk. The head moving mechanism moves the head in a radialdirection of the disk. The DC motor drives the head moving mechanism.The voltage generating means generates first and second voltages. Thefirst voltage has amplitude high enough to rotate the DC motor to such adegree as to get the head moved by the head moving mechanism, while thesecond voltage has the same polarity as the first voltage and has suchamplitude as to prevent the DC motor from rotating either in a backwarddirection due to a cogging torque or in a forward direction. The controlmeans controls the voltage generating means in such a manner that thevoltage generating means applies the second voltage to the DC motorafter having applied the first voltage to the DC motor.

[0058] In one preferred embodiment of the present invention, the controlmeans preferably gets a pulse voltage applied as the first voltage tothe DC motor. A pulse width T of the pulse voltage preferably satisfiest≦T≦5 t, where t is an electrical time constant of the DC motor.

[0059] In another preferred embodiment of the present invention, thefirst voltage is preferably three times or more as high as a minimumvoltage that needs to be applied to the DC motor for the head movingmechanism to overcome a load applied to the head and move the head.

[0060] In still another preferred embodiment, the control meanspreferably controls the voltage generating means in such a manner thatthe voltage generating means alternately applies the first and secondvoltages to the DC motor.

[0061] In yet another preferred embodiment, the voltage generating meansmay include a pulse width modulator and may generate the first andsecond voltages as effective pulse voltages to be output from the pulsewidth modulator.

[0062] Another preferred embodiment of the present invention provides amethod of driving a DC motor for the purpose of transmitting arotational force of the DC motor to an object, having a predeterminedmass, by way of a driving mechanism. The driving mechanism is coupled tothe DC motor and moves the object against a load applied to the object.The method includes the step of (a) generating first and secondvoltages. The first voltage has amplitude high enough to rotate the DCmotor, while the second voltage has the same polarity as the firstvoltage and has such amplitude as to prevent the DC motor from rotatingeither in a backward direction due to a cogging torque or in a forwarddirection. The method further includes the steps of (b) applying thefirst voltage to the DC motor and then (c) applying the second voltageto the DC motor.

[0063] In one preferred embodiment of the present invention, the step(b) preferably includes the step of applying a pulse voltage as thefirst voltage to the DC motor. A pulse width T of the pulse voltagepreferably satisfies t≦T≦5 t, where t is an electrical time constant ofthe DC motor.

[0064] In another preferred embodiment of the present invention, thestep (b) preferably includes the step of applying the first voltage,which is three times or more as high as a minimum voltage that needs tobe applied to the DC motor for the driving mechanism to overcome theload and move the object, to the DC motor.

[0065] In still another preferred embodiment, the method may furtherinclude the step of alternately applying the first and second voltagesto the DC motor.

[0066] Other features, elements, processes, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of preferred embodiments of the presentinvention with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0067]FIG. 1 schematically illustrates an optical disk drive including amotor controller according to a first specific preferred embodiment ofthe present invention.

[0068]FIG. 2 is a flowchart showing how the optical disk drive shown inFIG. 1 performs a sled control.

[0069]FIG. 3 is a timing diagram showing a relationship between the sledservo signal and the output voltages of the voltage generators in theoptical disk drive shown in FIG. 1.

[0070]FIG. 4 is a graph showing relationships between the firstvoltage/start voltage ratio and the magnitude of movement for twodifferent frictional loads.

[0071]FIG. 5 schematically illustrates an optical disk drive including amotor controller according to a second specific preferred embodiment ofthe present invention.

[0072]FIG. 6 is a flowchart showing how the optical disk drive shown inFIG. 5 performs a sled control.

[0073]FIG. 7 is a timing diagram showing a relationship between the sledservo signal and the output voltages of the voltage generators in theoptical disk drive shown in FIG. 5.

[0074]FIGS. 8A and 8B are schematic representations illustrating thestructure of a DC motor.

[0075]FIG. 9 is a graph showing a relationship between a cogging torquegenerated in a DC motor and the angle of rotation of the rotor.

[0076]FIG. 10A is a block diagram illustrating a configuration for themain section of a conventional optical disk drive; and

[0077]FIG. 10B is a plan view of the optical disk drive shown in FIG.10A.

[0078]FIGS. 11A, 11B and 11C are timing diagrams showing the respectivewaveforms of signals that are generated in the optical disk drive shownin FIG. 10A.

[0079]FIG. 12 is a flowchart showing how a sled control is carried outin the optical disk drive shown in FIG. 10A.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Embodiment 1

[0080] Hereinafter, a motor controller according to a first specificpreferred embodiment of the present invention will be described withreference to the accompanying drawings.

[0081]FIG. 1 schematically illustrates the main section of an opticaldisk drive 1 including the motor controller of the first preferredembodiment. The optical disk drive 1 may be used to read and/or writeinformation from/on an optical disk 2 such as a DVD-RAM, on which spiraltracks are formed.

[0082] The optical disk drive 1 includes a rotating/driving mechanismfor use to rotate the optical disk 2 that has been mounted thereon. Therotating/driving mechanism includes spindle motor 9, turntable 25 anddriver (not shown). The turntable 25 is fitted with the shaft 24 of thespindle motor 9 to mount the optical disk 2 thereon. The driver is usedto drive the spindle motor 9.

[0083] The optical disk drive 1 further includes optical head (oroptical pickup) 23, optical head moving mechanism 12 and DC motor 5. Theoptical head 23 is movable in the radial direction of the optical disk 2that has been mounted on the turntable 25. The optical head movingmechanism 12 is provided as a driving mechanism for moving the opticalhead 23 in the radial direction. The DC motor 5 is provided to drive theoptical head moving mechanism 12. To control the optical head 23 and theoptical head moving mechanism 12, the optical disk drive 1 furtherincludes control unit 19, tracking servo circuit 17, driver 21, sledservo circuit 22 and voltage generator circuit 13, which are stored in acasing (not shown). The radial direction of the optical disk 2 will beherein simply referred to as a “radial direction”. The tracking servocircuit 17, driver 21 and sled servo circuit 22 may be known componentsthat are normally included in a conventional disc drive for compactdiscs.

[0084] The optical head 23 includes laser diode 26 as a light source,divided photodiode 27 as a photodetector, objective lens (condenserlens) 28 and actuator 29.

[0085] The objective lens 28 is supported by a suspension spring (notshown), which is provided for the optical head 23 to apply an elasticforce thereto. The objective lens 28 is movable both in the radialdirection and in a direction parallel to the axis of rotation of theoptical disk 2 (i.e., along the optical axis of the objective lens 28)with respect to the optical head 23. When the objective lens 28 shiftsfrom its neutral position, an elastic force is applied from thesuspension spring to the objective lens 28 toward its neutral position.The direction parallel to the axis of rotation of the optical disk 2will be herein simply referred to as a “rotation axis direction”.

[0086] The actuator 29 includes a focus actuator (not shown) and atracking actuator 30. The focus actuator is provided to move theobjective lens 28 in the rotation axis direction with respect to theoptical head 23. The tracking actuator 30 is provided to move theobjective lens 28 in the radial direction from its home position that isdefined over the optical head 23. In this optical disk drive 1, variousconditions (e.g., the positions) of the tracking actuator 30 and thesuspension spring are adjusted so that the home position matches withthe neutral position.

[0087] While no voltage is applied to the tracking actuator 30, theobjective lens 28 is located at its home position due to the elasticforce applied from the suspension spring to the objective lens 28. Butwhen a predetermined voltage is applied to the tracking actuator 30 byway of the driver 21, the tracking actuator 30 moves the objective lens28 in the radial direction in accordance with the polarity and theamplitude of the voltage. Specifically, the polarity of the voltagedetermines the direction in which the objective lens 28 should move,while the amplitude of the voltage determines the distance for which theobjective lens 28 should move from its home position.

[0088] The optical head moving mechanism 12 includes nut piece 3, leadscrew 4, guide shaft 6, bearings 7, pinion 10, spur gear 11 andsupporting members (sliders) 31. The lead screw 4 is coupled togetherwith the spur gear 11 and supported in a rotatable state on the bearings7. The lead screw 4 has a screw pitch of about 4 mm. The bearings 7 arefixed on a chassis 8. The spur gear 11 engages with the pinion 10 thatis directly coupled to the output shaft of the DC motor 5. Accordingly,the number of revolutions of the DC motor 5 is decreased and thentransmitted to the lead screw 4.

[0089] The nut piece 3 is secured to the optical head 23 and engagedwith the thread groove of the lead screw 4. A spring (not shown) appliesan elastic force to one end of the nut piece 3. The supporting members31, which are supported by the guide shaft 6, are secured to the opticalhead 23 so that the optical head 23 can move smoothly when the nut piece3 moves with the lead screw 4 rotating.

[0090] The optical head moving mechanism 12 having such a structuretransforms the rotational motion of the DC motor 5 into a linear motion,thereby overcoming a load created by the mass of the optical head 23 andthe mechanism itself and moving the optical head 23 in the radialdirection of the optical disk 2. The deceleration ratio caused by thepinion 10 and the spur gear 11 is two. Accordingly, the optical head 23moves 2 mm per revolution of the DC motor 5. The maximum number ofrevolutions per minute of the DC motor 5 is 9,000 rpm. Accordingly, theoptical head 23 can be moved at a maximum velocity of 300 mm/s, therebyrealizing a high-speed seek operation. To achieve a positioning accuracyof ±20 μm as required for a DVD-RAM, the rotor of the DC motor 5 shouldstop at a step angle of ±3.6 degrees or less.

[0091] The DC motor 5 is a small-sized three-pole motor that usespermanent magnets as its fields. In this DC motor 5, its fields arepermanent magnets and require small spaces, and therefore, its rotor mayhave a relatively large diameter. Also, due to its small size, the DCmotor 5 can operate at a low voltage rather efficiently. However, such arotor has a great moment of inertia as already described for thebackground of the invention.

[0092] By using its brush and commutator, the DC motor 5 can rotate onlywith a DC power supplied and without using any special driver. Theaverage output torque per revolution (i.e., 360 degrees) of the DC motor5 is substantially proportional to the drive current. However, sincethis DC motor 5 switches the excitation by using mechanical contactpoints and also has a small number of poles, a relatively greatvariation should be caused in the output torque per revolution.Accordingly, if the DC motor 5 is rotated to just a small step angle of3.6 degrees, for example, then the torque will be variable significantlydepending on the angular position of the rotor. Nevertheless, eventaking the possible variation in the torque of the DC motor 5 itself,the frictional load or the load applied to the optical head movingmechanism 12 into account, the DC motor 5 can be started as intended byapplying a voltage of about 1.5 V thereto.

[0093] The voltage generator circuit 13 includes amplifier 18, reversecurrent checker 33, first voltage generator 14 and second voltagegenerator 15, and receives a timing signal and a polarity signal fromthe control unit 19 as will be described later. The first voltagegenerator 14 includes a voltage selector 16 and is connected to a 5 Vpower supply (not shown). This optical disk drive 1 is also driven by a5 V power supply, and therefore, a supply voltage of 5 V is actually thehighest voltage and most likely used for the optical disk drive 1.

[0094] The voltage selector 16 may be implemented as a semiconductordevice, for example, and turns ON or OFF in response to the timingsignal supplied from the control unit 19. When the voltage selector 16turns ON, the power supply is directly connected to the DC motor 5 vialow impedance. Thus, a voltage approximately equal to the supply voltageof 5 V is applied to the DC motor 5. The voltage selector 16 actuallyincludes a plurality of semiconductor devices so as to invert thepolarity of the voltage, responsive to the polarity signal supplied fromthe control unit 19, when the optical head 23 should be transported inthe opposite direction. Since the voltage selector 16 is made up ofsemiconductor devices, the voltage selector 16 in the ON state has aslight ON-state resistance. For that reason, the output voltage of thefirst voltage generator 14 is not quite equal to the supply voltage butthe difference is a negligible one. Thus, the output voltage of thefirst voltage generator 14 is herein supposed to be 5 V.

[0095] The actual output voltage of the first voltage generator 14 ispreferably at least three times as high as a minimum voltage (e.g., 1.5V) that needs to be applied to the DC motor 5 to start it. The formervoltage will be herein referred to as a “first voltage”. By applying thefirst voltage to the DC motor 5, the DC motor 5 can rotate to such adegree as to get the optical head 23 moved by the optical head movingmechanism 12.

[0096] The second voltage generator 15 generates a second voltage, whichis smaller in amplitude than the first voltage. The second voltage hassuch amplitude as to prevent the DC motor 5 from rotating either in thebackward direction due to its own cogging torque or in the forwarddirection by overcoming the load (i.e., the optical head movingmechanism 12) applied thereto. As used herein, the “forward direction”refers to the direction that is defined by the polarity of the firstvoltage while the “backward direction” refers to the direction oppositeto the former direction. In this preferred embodiment, the secondvoltage may be about 0.2 V, for example. The second voltage generator 15also receives the polarity signal from the control unit 19 so as toinvert the polarity of its second voltage responsive to the polaritysignal.

[0097] The output voltage of the second voltage generator 15 is input tothe amplifier 18. The amplifier 18 is a current amplifier and outputsthe same voltage as its input voltage at an impedance low enough todrive the DC motor 5. The output of the amplifier 18 is supplied to theDC motor 5 by way of the reverse current checker 33.

[0098] The reverse current checker 33 may be a diode, for example, andprevents the output current of the first voltage generator 14 fromflowing in the reverse direction toward the amplifier 18 while thevoltage selector 16 is ON. By providing this reverse current checker 33,the second voltage of 0.2 V can be applied from the second voltagegenerator 15 to the DC motor 5 while the voltage selector 16 is OFF, andthe first voltage of 5 V can be applied from the first voltage generator14 to the DC motor 5 while the voltage selector 16 is ON.

[0099] The control unit 19 may be implemented as a microprocessor (orCPU) with memories and performs an overall control on the optical head23, DC motor 5, spindle motor 9, tracking servo circuit 17, sled servocircuit 22 and other components of the optical disk drive 1. The controlunit 19 includes a timing generator 20, which may be implemented as atimer, for example. The timing generator 20 outputs the timing signal tothe voltage selector 16, thereby turning the voltage selector 16 ON andgetting the supply voltage of 5 V applied as a pulse voltage to DC motor5 for just a short period of time.

[0100] Hereinafter, it will be described how the optical disk drive 1operates. The optical disk drive 1 moves the optical head 23 to a targettrack (or target address), where the optical disk drive 1 reads and/orwrites information from/on the optical disk 2 while performing focuscontrol, tracking control, sled control, rotational velocity control andother controls. Among these various types of controls, the focus androtational velocity controls of the optical disk drive 1 may be carriedout as in the conventional optical disk drive, and the descriptionthereof will be omitted herein.

[0101] Thus, the tracking control performed by the optical disk drive 1will be described first. When the optical disk drive 1 performs thetracking control, first, the laser diode 26 of the optical head 23 emitsa laser beam toward the optical disk 2. The laser beam that has beenreflected from the optical disk 2 is received at the divided photodiode27. Then, the photodiode 27 converts the received laser beam into avoltage photoelectrically, thereby outputting the resultant voltagesignal to the tracking servo circuit 17. In response to the voltagesignal that has been supplied from the photodiode 27, the tracking servocircuit 17 generates a tracking error signal TE as a voltage signal. Thetracking error signal TE represents how much the objective lens 28 isshifted from the center of the target track in the radial direction.

[0102] The tracking servo circuit 17 also generates a tracking servosignal TS as another voltage signal by performing predetermined signalprocessing, including phase inversion and amplification, on the trackingerror signal TE. The tracking servo signal TS is generated to drive thetracking actuator 29 in such a manner that the objective lens 28 movesto the center of the target track (i.e., so that the tracking errorsignal TE has a zero level). That is to say, the tracking servo signalTS may also be regarded as a drive voltage for the tracking actuator 29.Accordingly, the voltage value of the tracking servo signal TScorresponds to the shift of the objective lens 28 from the center of thetarget track, while the sign of the tracking servo signal TS representsthe direction of the shift. The tracking servo signal TS is input notjust to the tracking actuator 29 via the driver 21 but also to the sledservo circuit 22.

[0103] The tracking actuator 29 is driven responsive to the trackingservo signal TS. By driving the tracking actuator 29, the objective lens28 can be moved toward the center of the target track, which operationis normally called a “tracking servo”.

[0104] However, if the objective lens 28 is moved continuously bydriving the tracking actuator 29, then the objective lens 28 will soonreach a limit of its movable range inside the optical head 23 and willnot be able to move and follow the track anymore. Likewise, the trackingactuator 29 cannot move the objective lens 28 beyond its limit, either.Thus, by driving the DC motor 5, the optical head 23 is moved in thedirection in which the objective lens 28 has moved so that the objectivelens 28 returns to its home position. This is so-called “sled control”.

[0105] Normally, in a read-only CD-ROM drive, the range in which thetracking actuator 29 can make the objective lens 28 follow the targettrack is defined approximately as ±200 μm. If the target track islocated outside of this range, then the sled control is carried out tomove the optical head 23 and to make the target track fall within thisrange. In this case, the positioning accuracy of the optical head 23with respect to the target track should be within ±200 μm. As for aDVD-RAM, which has a greater storage capacity than a CD-ROM and on whichinformation can be written, this range is even narrower and typicallywithin ±20 μm. Accordingly, the positioning accuracy of the optical head23 with respect to the target track should also be within ±20 μm.

[0106] According to preferred embodiments of the present invention, theDC motor 5 is driven by a unique method to perform the sled control.Hereinafter, it will be described in detail how to carry out the sledcontrol in this preferred embodiment of the present invention.

[0107] Specifically, the sled servo circuit 22 performs predeterminedsignal processing, including removal of high frequency components andamplification, on the tracking servo signal TS that has been suppliedfrom the tracking servo circuit 17, thereby generating a sled servosignal SS. That is to say, the voltage value of the sled servo signal SScorresponds to the shift of the objective lens 28 from the center of thetarget track, while the sign of the sled servo signal SS represents thedirection of the shift.

[0108] The sled servo signal SS represents a voltage corresponding tothe drive voltage of the tracking actuator 29. Therefore, if the voltagerepresented by the sled servo signal SS has an absolute value greaterthan a certain reference value, then the sled control is carried out tomove the optical head 23.

[0109] For that purpose, the sled servo signal SS is input to thecontrol unit 19 to monitor the absolute value of the voltage representedby the sled servo signal SS. If the voltage represented by the sledservo signal SS has an absolute value greater than the reference value,then the timing generator 20 outputs the timing signal to the voltageselector 16 for a short time. As a result, the first voltage of 5 V isapplied from the first voltage generator 14 to the DC motor 5 while thetiming signal is being output. On the other hand, if the voltagerepresented by the sled servo signal SS has an absolute value equal toor smaller than the reference value, then no timing signal is output andno voltage is applied from the first voltage generator 14 to the DCmotor 5. Instead, the second voltage of 0.2 V is applied from the secondvoltage generator 15 to the DC motor 5.

[0110] In this manner, if the absolute value of the sled servo signal SSis equal to or smaller than the reference value, the second voltage of0.2 V is applied to the DC motor 5. However, once the absolute value ofthe sled servo signal SS exceeds the reference value, the first voltageof 5 V is applied to the DC motor 5 for a short time (e.g., 300 μs).When the timing signal has been output, the second voltage of 0.2 V willbe applied to the DC motor 5 again.

[0111] The short time interval T during which the timing signal isoutput and the electrical time constant t of the DC motor 5 preferablysatisfy t≦T≦5 t. The reason is as follows. Specifically, if the timeinterval T of the timing signal is shorter than the electrical timeconstant t of the DC motor 5, then the first voltage is applied to theDC motor 5 for too short a time to drive the DC motor 5. On the otherhand, if the time interval T of the timing signal is more than 5 timesas long as the electrical time constant t of the DC motor 5, then thestep angle of the DC motor 5 will be too large to control the opticalhead 23 precisely. In this preferred embodiment, the electrical timeconstant t of the DC motor 5 is 100 μs. Thus, the time interval T of thetiming signal is set to 300 μs, which is three times as long as theelectrical time constant t of the DC motor 5.

[0112] Next, it will be described in further detail with reference toFIG. 2 how to perform the sled control using the sled servo signal SS.FIG. 2 is a flowchart showing how the control unit 19 of the opticaldisk drive 1 operates.

[0113] First, in Step 41, the control unit 19 determines whether or notthe absolute value of the sled servo signal has exceeded the referencevalue. As described above, the voltage value of the sled servo signal SSrepresents how much and in which direction the objective lens 28 isshifted from its home position. The reference value is defined so as tocorrespond to a shift that falls within the range in which the trackingactuator 29 can make the objective lens 28 follow the track. If theanswer to the query of Step 41 is NO, Step 41 is carried out again.

[0114] On the other hand, if the answer to the query of Step 41 is YES,then the control unit 19 detects the polarity of the sled servo signalSS in Step 42. If the polarity of the sled servo signal SS is foundpositive, then the control unit 19 supplies the polarity signal to thefirst voltage generator 14, thereby defining the output of the firstvoltage generator 14 as positive in Step 43. Also, the control unit 19instructs the timing generator 20 to output the timing signal to thefirst voltage generator 14, thereby making the voltage generator circuit13 output the positive first voltage as a pulse voltage, which isapplied to the DC motor 5. Next, in Step 44, the control unit 19 appliesthe polarity signal to the second voltage generator 15 to define theoutput of the second voltage generator 15 as positive. That is to say,as soon as the Step 43 of outputting the first voltage is finished, thesecond voltage having the same polarity as the first voltage will beoutput from the second voltage generator 15 in Step 44. Subsequently,when a predetermined amount of standby time provided for Step 47 passes,the processing will return to Step 41 again.

[0115] On the other hand, if the polarity of the sled servo signal SS isfound negative in Step 42, then the control unit 19 supplies thepolarity signal to the first voltage generator 14, thereby defining theoutput of the first voltage generator 14 as negative in Step 45. Also,the control unit 19 instructs the timing generator 20 to output thetiming signal to the first voltage generator 14, thereby making thevoltage generator circuit 13 output the negative first voltage as apulse voltage, which is applied to the DC motor 5. Next, in Step 46, thecontrol unit 19 applies the polarity signal to the second voltagegenerator 15 to define the output of the second voltage generator 15 asnegative. That is to say, as soon as the Step 45 of outputting the firstvoltage is finished, the second voltage having the same polarity as thefirst voltage will be output from the second voltage generator 15 inStep 46. Subsequently, when a predetermined amount of standby timeprovided for Step 47 passes, the processing will return to Step 41again.

[0116]FIG. 3 is a graph showing a relationship between the sled servosignal and the voltage applied to the DC motor 5. Hereinafter, thevoltages output from the voltage generator circuit 13 in response to thesled servo signal will be described with reference to FIGS. 2 and 3.

[0117] In a time interval T1 just after the control unit 19 has startedto perform the sled control, the sled servo signal SS has a positivevalue smaller than the reference value. When the sled servo signal SS ispositive, the objective lens 28 has been shifted toward the outerperiphery of the optical disk 2. During this interval T1, the controlunit 19 performs Step S41 repeatedly. As already described withreference to FIG. 1, while the first voltage generator 14 of the voltagegenerator circuit 13 is not outputting the first voltage, the secondvoltage generator 15 thereof always outputs the second voltage.Accordingly, the voltage generator circuit 13 is outputting the secondvoltage of +0.2 V in the interval T1 in the example shown in FIG. 3. Inthis interval T1, however, as long as no voltage is output from thefirst voltage generator 14, the voltage generator circuit 13 may outputany voltage, e.g., −0.2 V or 0 V. In any case, the absolute value of theoutput voltage is smaller than that of the minimum voltage of 1.5 V thatneeds to be applied to the DC motor 5 to start it. Thus, the DC motor 5remains stopped.

[0118] In the next interval T2, the sled servo signal has a positivevalue that is greater than the reference value. Accordingly, the controlunit 19 performs Steps 41, 42 and 43 shown in FIG. 2, thereby making thefirst voltage generator 14 generate and output the positive firstvoltage of +5 V. Thus, the voltage generator circuit 13 applies thefirst voltage as a pulse voltage P1 to the DC motor 5 for 300 μs. Thepulse voltage P1 is sufficiently greater than the minimum voltage of 1.5V that needs to be applied to the DC motor 5 to start it. Accordingly,the DC motor 5 starts to rotate. Thereafter, the control unit 19performs Steps 44 and 47. That is to say, as soon as the first voltagehas been output, the positive second voltage of +0.2 V will be outputfrom the second voltage generator 15. In this manner, the instant thepulse voltage P1 has been output from the voltage generator circuit 13,the positive second voltage will be applied to the DC motor 5.

[0119] When the DC motor 5 has rotated to a predetermined angleresponsive to the pulse voltage P1, the DC motor 5 will stop rotating.At that point in time, the rotor of the DC motor 5 might rotate in thebackward direction due to the cogging torque of the DC motor 5. In thispreferred embodiment, however, the second voltage, having the samepolarity as the pulse voltage P1, is continuously applied to the DCmotor 5. Accordingly, it is possible to prevent the rotor from rotatingin the backward direction. As a result, the DC motor 5 rotates to atleast the angle corresponding to the drive pulse voltage P1 applied andthen stops.

[0120] Since the positive voltage is applied to the DC motor 5, the DCmotor 5 rotates in such a direction as to get the optical head 23 movedtoward the outer periphery of the optical disk 2. As a result, theposition of the objective lens 28 is adjusted and the voltage value ofthe sled servo signal SS decreases to less than the reference value.Accordingly, in the next interval T3, the control unit 19 repeatedlyperforms Step 41 again. In the meantime, the voltage generator circuit13 is outputting the positive second voltage of +0.2 V.

[0121] In the next interval T4, the sled servo signal SS has a positivevalue greater than the reference value again. Accordingly, as in theinterval T2, the voltage generator circuit 13 applies the positive firstvoltage as a pulse voltage P2 to the DC motor 5. As soon as the pulsevoltage P2 has been applied, the positive second voltage will beapplied. Thus, the DC motor 5 rotates to the predetermined angle andthen stops without rotating in the backward direction. The optical head23 also moves outward. In this interval T4, however, the position of theobjective lens 28 is still shifted from its home position. For thatreason, the voltage value of the sled servo signal SS is greater thanthe reference value. Accordingly, Steps 41, 42, 43 and 44 are repeatedlyperformed, the positive first voltage is applied as a pulse voltage P3to the DC motor 5 and then the positive second voltage of +0.2 V isapplied to the DC motor 5. As a result, the optical head 23 moves againand the voltage value of the sled servo signal SS decreases to less thanthe reference value.

[0122] In the next interval T5, the absolute value of the sled servosignal SS is less than the reference value but its sign changes frompositive into negative. However, since the absolute value of the sledservo signal SS is less than the reference value, the control unit 19repeatedly performs Step 41. In the meantime, the voltage generatorcircuit 13 is outputting the positive second voltage.

[0123] In the next interval T6, the sled servo signal SS has a negativevoltage having an absolute value greater than the reference value. Whenthe sled servo signal SS is negative, the objective lens 28 has beenshifted toward the inner periphery of the optical disk 2. Accordingly,the control unit 19 performs Steps 41, 42 and 45 shown in FIG. 2,thereby making the first voltage generator 14 generate and output thenegative first voltage of −5 V. Thus, the voltage generator circuit 13applies the negative first voltage as a pulse voltage P4 to the DC motor5 for 300 μs. The pulse voltage P4 has an absolute value that issufficiently greater than that of the minimum voltage of 1.5 V thatneeds to be applied to the DC motor 5 to start it. Accordingly, the DCmotor 5 rotates. In this interval T6, however, the DC motor 5 rotates inthe opposite direction to that of the interval T2. Thereafter, thecontrol unit 19 performs Steps 46 and 47. That is to say, as soon as thefirst voltage has been output, the negative second voltage of −0.2 Vwill be generated and output from the second voltage generator 15. Inthis manner, the instant the pulse voltage P4 has been output from thevoltage generator circuit 13, the negative second voltage will beapplied to the DC motor 5.

[0124] As soon as the pulse voltage P4 has been applied to the DC motor5, the second voltage having the same polarity as the pulse voltage P4is applied to the DC motor 5. Accordingly, the DC motor 5 does notrotate in the opposite direction due to the cogging torque but rotatesexactly to the angle corresponding to the drive pulse voltage P4 andthen stops. As the DC motor 5 rotates, the optical head 23 moves towardthe inner periphery of the optical disk 2. As a result, the position ofthe objective lens 28 is adjusted and the absolute value of the sledservo signal SS decreases to less than the reference value. Thus, in thenext interval T7, the control unit 19 repeatedly performs Step 41 again.In the meantime, the voltage generator circuit 13 is outputting thenegative second voltage of −0.2 V.

[0125] In this preferred embodiment, the optical head 23 moves about 10μm responsive to each of the pulse voltages P1 through P4. Thus, theshift of the objective lens 28 from its home position is decreased by 10μm per pulse so as to fall within the ±20 μm range in the end. That isto say, the relative position of the target track can be moved to withinthe range of ÷20 μm in which the tracking actuator 29 can make theobjective lens 28 follow the track.

[0126] It should be noted that in an interval between the application ofa first voltage as a pulse voltage P1, P2, . . . , etc. and the nextapplication of another first voltage as a pulse voltage P2, P3, . . . ,etc., the second voltage having the same polarity as the previous firstvoltage is preferably applied continuously as shown in FIG. 3. Thereason is as follows. In this interval, a cogging torque may have beengenerated in the DC motor 5 in such a direction as to rotate the DCmotor 5 to the backward direction. Accordingly, by applying the secondvoltage to the DC motor 5, such unwanted reversal can be prevented. Inother words, if the application of the second voltage was stopped at anytime between the pulse voltages P1 and P2, for example, the DC motor 5might start to rotate in the backward direction at that point in time.

[0127] However, unless the DC motor 5 rotates to the backward direction,the second voltage does not always have to be applied continuouslythrough the interval between two first voltages. For example, theapplication of the second voltage may be suspended for too short aperiod of time to allow the DC motor 5 to reverse its direction ofrotation. Or the application of the second voltage may also bediscontinued by applying a third voltage, also having the same polarityas the first and second voltages and such amplitude as to prevent the DCmotor 5 from rotating either in the backward direction or the forwarddirection, to the DC motor 5 during that interval.

[0128] In FIG. 3, the pulse voltages 151 and 152 (see FIG. 11C) appliedto the DC motor of the conventional disk drive 101 (see FIG. 10) areindicated by the dashed line. As shown in FIG. 3, the first and secondpulse voltages 151 and 152 applied to the DC motor of the conventionaldisk drive to drive it have a pulse width on the order of severalmilliseconds, which is far greater than that of the pulse voltages foruse in the preferred embodiments of the present invention. This isbecause the conventional disk drive does not intend to control the DCmotor at such a small step angle as that used in the preferredembodiments of the present invention. Also, the second pulse voltage 152is applied for a purpose that is totally different from that of thesecond voltage applied in the preferred embodiments of the presentinvention. Specifically, in the prior art, the second pulse voltage 152is applied as a braking pulse for preventing the rotor of the DC motorthat is driven responsive to the first pulse voltage 151 from rotatingexcessively due to its moment of inertia.

[0129] As described above, in the prior art, even if the pulse width ofthe first and second pulse voltages 151 and 152 is decreased to reducethe step angle of the DC motor, the DC motor will not rotate due to itsreversal resulting from the cogging torque and the optical head 103 justvibrates. Thus, according to the conventional technique, the DC motorcannot be controlled at a small step angle.

[0130] In contrast, according to the preferred embodiments of thepresent invention, a first voltage is applied as a pulse to the DC motorto drive it, and then a second voltage having the same polarity as thefirst voltage is applied thereto to avoid the reversal resulting fromthe cogging torque. The second voltage does not have so great amplitudeas to rotate the DC motor 5 but has amplitude high enough to avoid thereversal due to the cogging torque. In this manner, the reversal due tothe cogging torque is avoidable by getting the second voltage appliedfrom the second voltage generator 15 to the DC motor. Accordingly, evenif the first voltage is applied for just a short time, the DC motor 5still can rotate constantly without reversing its direction. As aresult, the pulse width of the first voltage applied can be shortenedsufficiently and the DC motor 5 can be controlled at a very small stepangle.

[0131] In addition, when the first voltage is applied for a sufficientlyshort time, the rotor of the DC motor is not accelerated excessively.Thus, even if the rotor has a great moment of inertia, the inertialforce of the rotor can be reduced. Accordingly, the rotor can be stoppedjust as intended only by the frictional force after having rotated to avery small angle. As a result, the DC motor can be controlledconstantly. Furthermore, unlike the prior art, no reversing pulse isused in the preferred embodiments of the present invention. Thus, evenif the driving mechanism being driven by the DC motor has a load in thebackward direction that is far smaller than a load in the forwarddirection, no instability will be observed in the operation of thedriving mechanism since no reversing pulse is used. Consequently, the DCmotor is controllable with high positioning accuracy.

[0132] Furthermore, according to the preferred embodiments of thepresent invention, the first voltage is applied by directly switchingthe supply voltage of 5 V. Thus, the first voltage can be three times ormore as high as the minimum voltage of 1.5 V that needs to be applied tothe DC motor 5 to start it.

[0133] Next, it will be described with reference to FIG. 4 what effectsare achieved by applying such a high voltage as the first voltage. FIG.4 is a graph showing relationships between the first voltage and themagnitude of movement of the optical head for two different frictionalloads. In FIG. 4, the abscissa represents a ratio of the first voltageto the “start voltage”. As used herein, the “start voltage” means theminimum voltage that needs to be applied to the DC motor 5 for theoptical head moving mechanism 12 to overcome a frictional load appliedto the optical head 23 and move the head 23 in the optical disk drive ofthis preferred embodiment. That is to say, in FIG. 4, the first voltageis changed with respect to the constant start voltage. Also, a pulsewidth that results in an appropriate magnitude of movement is definedfor each first voltage. And for each pulse width obtained in thismanner, respective magnitudes of movement are plotted as ordinates inFIG. 4 with respect to two different frictional loads.

[0134] The curves shown in FIG. 4 respectively show a situation wherethe frictional load is doubled (as indicated by solid circles ) and asituation where the frictional load is halved (as indicated by the solidsquares ▪) compared to a normal state value. The frictional load willusually change within such a range due to various factors includingdeterioration with time, abrasion of the driving mechanism andtemperature variations.

[0135] As can be clearly seen from FIG. 4, where the first voltage/startvoltage ratio is less than two, these two curves are far apart from eachother. That is to say, if the frictional load is relatively small, themagnitude of movement is excessively great. On the other hand, if thefrictional load is relatively large, the magnitude of movement isextremely small.

[0136] In contrast, if the first voltage/start voltage ratio is two ormore, these two curves approach to each other. And when the firstvoltage/start voltage ratio is three or more, these two curvessubstantially match with each other. As can be seen from FIG. 4, if thefirst voltage/start voltage ratio is two or less, the variation infrictional load changes the magnitude of movement of the optical headsignificantly. However, if the first voltage/start voltage ratio isthree or more, then the magnitude of movement of the optical headremains substantially the same irrespective of the variation infrictional load.

[0137] In the preferred embodiment described above, the first voltage isthree times or more as high as the start voltage. Accordingly,irrespective of the variation in frictional load, the optical head canbe moved just as intended. Thus, even if the optical head has moved toless than the predetermined magnitude of movement responsive to a singledrive pulse and if the load at the new position is greater than the loadat the original position due to a cogging torque, the optical head stillcan be moved to the intended position by applying a drive pulse voltageof the same amplitude again. As a result, the optical head iscontrollable constantly without causing any failure of control.

[0138] As described above, according to the preferred embodiment of thepresent invention, the second voltage generated by the second voltagegenerator 15 prevents the DC motor 5 from rotating in the backwarddirection due to a cogging torque. Thus, even if the first voltage isapplied to the DC motor 5 for just a short time to drive it, the DCmotor 5 still can be driven constantly. For that reason, the pulse widthof the first voltage can be shortened and the step angle of the DC motor5 can be reduced sufficiently. In addition, the DC motor 5 can berotated to a very small step angle and then stopped only by thefrictional force. Accordingly, the DC motor 5 can be operated just asintended without losing stability of control. As a result, sufficientlyhigh positioning accuracy is achieved.

[0139] Furthermore, the first voltage is three times or more as high asthe start voltage. Accordingly, even if the frictional load has changedsignificantly, the DC motor 5 still can be driven constantly. That is tosay, even in a situation where the DC motor 5 has moved to less than thepredetermined magnitude responsive to a single drive pulse and thefrictional load has increased considerably, the DC motor 5 still can bedriven constantly, moved to the predetermined magnitude, and controlledjust as intended by applying the same pulse to the DC motor 5 again.

Embodiment 2

[0140] Hereinafter, a motor controller according to a second specificpreferred embodiment of the present invention will be described withreference to the accompanying drawings.

[0141]FIG. 5 schematically illustrates the main section of an opticaldisk drive 1 a including a motor controller according to the secondpreferred embodiment. In FIG. 5, each component of the optical diskdrive la having substantially the same function as the counterpart ofthe optical disk drive 1 is identified by the same reference numeral.

[0142] Unlike the optical disk drive 1 of the first preferred embodimentdescribed above, the optical disk drive la of the second preferredembodiment includes a control unit 19 a having a timing changer 32.Thus, the following description of the second preferred embodiment willbe focused on the operation of the control unit 19 a.

[0143] As shown in FIG. 5, the control unit 19 a includes the timinggenerator 20 and the timing changer 32. The timing changer 32 changesthe interval during which the timing signal is generated by the timinggenerator 20 (i.e., the interval during which the first voltage isoutput) approximately proportionally to the absolute value of the sledservo signal. As described above, the first voltage is output when theabsolute value of the sled servo signal is greater than a certainreference value. Accordingly, the shortest length of the first voltageoutput interval is a predetermined value corresponding to the referencevalue. In this preferred embodiment, the shortest length is alsosupposed to be 300 μs as in the first preferred embodiment. However, aslong as the shortest length T of the first voltage output interval andthe time constant t of the DC motor 5 satisfy t≦T≦5 t, the shortestlength T may be changed into any other value. If the absolute value ofthe sled servo signal is still greater than the reference value evenafter the first voltage has been applied to the DC motor 5 by the methodto be described later, then the timing changer 32 changes the timingsignal interval so that the length of the first voltage output intervalas defined by the timing signal is approximately proportional to theabsolute value of the sled servo signal.

[0144] Hereinafter, it will be described with reference to FIG. 6 howthe control unit 19 a of the optical disk drive la carries out a sledcontrol. FIG. 6 is a flowchart showing how the control unit 19 aperforms its sled control operation.

[0145] First, in Step 61, the control unit 19 a determines whether ornot the absolute value of the sled servo signal is greater than areference value. As already described for the first preferredembodiment, the voltage value of the sled servo signal SS represents howmuch the objective lens 28 is shifted from its home position. Thereference value is defined so as to correspond to a shift that fallswithin the range in which the tracking actuator 29 can make theobjective lens 28 follow the track.

[0146] If the answer to the query of Step 61 is NO, Step 61 is carriedout again. On the other hand, if the answer to the query of Step 61 isYES, then the control unit 19 a detects the polarity of the sled servosignal SS in Step 62.

[0147] If the polarity of the sled servo signal SS is found positive inStep 62, then the control unit 19 a supplies the polarity signal to thefirst voltage generator 14, thereby defining the output of the firstvoltage generator 14 as positive in Step 63. Also, the timing generator20 outputs the timing signal to the first voltage generator 14, therebymaking the voltage generator circuit 13 output the positive firstvoltage as a pulse voltage, which is applied to the DC motor 5. Next, inStep 64, the control unit 19 a applies the polarity signal to the secondvoltage generator 15 to define the output of the second voltagegenerator 15 as positive. That is to say, as soon as the Step 63 ofoutputting the first voltage is finished, the second voltage having thesame polarity as the first voltage will be output from the secondvoltage generator 15 in Step 64. Subsequently, when a predeterminedamount of standby time provided for Step 67 passes, the control unit 19a determines again in Step 68 whether or not the absolute value of thesled servo signal is still greater than the reference value.

[0148] On the other hand, if the polarity of the sled servo signal SS isfound negative in Step 62, then the control unit 19 a supplies thepolarity signal to the first voltage generator 14, thereby defining theoutput of the first voltage generator 14 as negative in Step 65. Also,the timing generator 20 outputs the timing signal to the first voltagegenerator 14, thereby making the voltage generator circuit 13 output thenegative first voltage as a pulse voltage, which is applied to the DCmotor 5. Next, in Step 66, the control unit 19 a applies the polaritysignal to the second voltage generator 15 to define the output of thesecond voltage generator 15 as negative. That is to say, as soon as theStep 65 of outputting the first voltage is finished, the second voltagehaving the same polarity as the first voltage will be output from thesecond voltage generator 15 in Step 66. Subsequently, when thepredetermined amount of standby time provided for Step 67 passes, thecontrol unit 19 a determines again in Step 68 whether or not theabsolute value of the sled servo signal is still greater than thereference value.

[0149] If the absolute value of the sled servo signal is found stillgreater than the reference value in Step 68, then the timing changer 32changes, in Step 69, the interval during which the timing generator 20generates the timing signal in accordance with the absolute value of thesled servo signal. Thereafter, the processing returns to Step 62.

[0150] The same processing steps will be repeatedly performed from Step62 on. In the meantime, the first voltage is applied to the DC motor 5for a length of time that is proportional to the absolute value of thesled servo signal. On the other hand, if the absolute value of the sledservo signal is found smaller than the reference value in Step 68, thenthe interval during which the timing generator 20 generates the timingsignal is reset in Step 70 and then the processing returns to Step 61.Once the timing signal interval is reset in Step 70, the first voltagewill be applied to the DC motor 5 for 300 μs, which is the predeterminedinitial value, in Step 63 or 65 to be carried out after the Step 70.

[0151]FIG. 7 is a graph showing a relationship between the sled servosignal and the voltage applied to the DC motor 5. As shown in FIG. 7,the sled servo signal has a positive value, of which the absolute valueis greater than the reference value, in interval T1. Thus, as describedfor the first preferred embodiment, a drive pulse voltage P5, havingamplitude equal to that of the positive first voltage and a pulse widthof 300 μs, is applied to the DC motor 5, thereby moving the optical head23. As a result, the shift of the objective lens 28 from its homeposition falls within a predetermined range and the sled servo signalcomes to have an absolute value smaller than the reference value.

[0152] In the next interval T2, the sled servo signal has a positivevalue having an absolute value that is far greater than the referencevalue. Accordingly, the absolute value of the sled servo signal cannotbe decreased to less than the reference value just by applying a drivepulse voltage P6, having amplitude equal to that of the positive firstvoltage, to the DC motor 5 for 300 μs a single time and getting theoptical head 23 moved. Thus, in that case, the timing changer 32 changesthe timing signal interval proportionally to the absolute value of thesled servo signal in Step 69. Subsequently, after the sign of the sledservo signal is found positive in Step 62, another positive drive pulsevoltage P7 is applied to the DC motor 5 in Step 63. The amplitude of thedrive pulse voltage P7 is equal to that of the positive first voltage(i.e., the drive pulse voltage P6) but the pulse width thereof isproportional to the absolute value of the sled servo signal and isgreater than that of the drive pulse voltage P6. Thus, by applying sucha drive pulse voltage P7 to the DC motor 5, the optical head 23 can bemoved for a longer distance compared to the situation where the drivepulse voltage P6 is applied to the DC motor 5. As a result, theobjective lens 28 can be moved more quickly within a predetermineddistance range as defined with respect to its home position.

[0153] In this manner, according to this preferred embodiment, if theoptical head cannot be moved for a predetermined distance just byapplying the first voltage to the DC motor a single time, then the firstvoltage of the same amplitude is applied to the DC motor again for alonger time, thereby moving the optical head for a longer distance. As aresult, the optical head can be moved to the target position morequickly.

[0154] In the first and second preferred embodiments described above,the voltage generator circuit continuously applies direct currentvoltages as the first and second voltages for respectively predeterminedamounts of time. Alternatively, the voltage generator circuit mayinclude a pulse width modulator (PWM) for outputting the first andsecond voltages as effective pulse voltages by controlling a duty ratioof a pulse voltage.

[0155] Also, in the preferred embodiments described above, the negativeand positive first voltages have mutually inverse polarities and thesame absolute value and the negative and positive second voltages alsohave mutually inverse polarities and the same absolute value. However,the negative first and second voltages do not have to have the sameabsolute values as the positive first and second voltages, respectively.For example, the absolute value of the positive first voltage may begreater than that of the negative first voltage. That is to say, themovement of an object to be driven (e.g., the optical head 23) may becontrolled by moving the object for mutually different distances in oneand opposite directions.

[0156] Also, in the preferred embodiments described above, the secondvoltage is output continuously from the voltage generator circuit exceptthe intervals in which the first voltage is output. However, if theoptical disk drive is kept OFF for a predetermined amount of time ormore, then the second voltage may be decreased or set to zero.

[0157] Furthermore, in the optical disk drive according to the preferredembodiments described above, the second voltage output from the voltagegenerator circuit 13 is predefined. Optionally, every time the opticaldisk drive is powered, the optical disk drive may perform learning todefine the second voltage in the following manner. First, after theoptical disk drive has been powered, the control unit gets the firstvoltage applied to the DC motor, thereby moving the optical head 23.Next, either 0 V or a predetermined second voltage is applied to the DCmotor to see whether or not the optical head 23 moves. Thereafter, thesecond voltage is increased gradually to find a second voltage value atwhich the optical head 23 no longer moves in the backward direction. Thesecond voltage value found in this manner may be defined as the secondvoltage.

[0158] Furthermore, in the second preferred embodiment described above,the timing signal interval is changed with the absolute value of thesled servo signal. Alternatively, every time the timing signal isapplied as pulse, the timing signal interval may also be simplyincreased by a predetermined amount of time.

[0159] Furthermore, in the first and second preferred embodiments, thepresent invention has been described as being applied to an optical diskdrive. However, the present invention is also applicable to variousother types of motor controllers including: a disk drive that uses a DCmotor and performs read and/or write operations non-optically; andprinter, photocopier, facsimile and robot that require highly accuratepositioning control.

[0160] A motor controller according to various preferred embodiments ofthe present invention described above applies a first voltage, which ishigh enough for a driving mechanism to overcome a load applied to anobject and drive the object, to a DC motor, thereby moving the object.Immediately after that, the motor controller applies a second voltage tothe DC motor, thereby preventing the DC motor from rotating either inthe backward direction due to its own cogging torque or in the forwarddirection. Accordingly, the first voltage may be applied to the DC motorfor a sufficiently short time to drive the object. Thus, the presentinvention provides a motor controller that achieves high positioningaccuracy and operates constantly.

[0161] In addition, in the motor controller according to the preferredembodiments of the present invention, the first voltage is three timesor more as high as the minimum voltage that needs to be applied to theDC motor to start it. Accordingly, even if the object of control has notmoved for the predetermined distance just by applying the first voltageto the DC motor a single time and if the frictional load has increasedsignificantly, the frictional load applied to the object can be overcomeand the object can be moved to the target position by applying the samefirst voltage to the DC motor again. Thus, the present inventionprovides a motor controller that can perform a stabilized controloperation without causing any failure of control.

[0162] Furthermore, in the motor controller according to the preferredembodiments of the present invention described above, when the samefirst voltage is applied again to the DC motor, that first voltage isapplied for a longer time to move the object of control for a longerdistance. Thus, the motor controller can move the object for thepredetermined distance more quickly by applying the first voltage of thesame amplitude to the DC motor again.

[0163] While the present invention has been described with respect topreferred embodiments thereof, it will be apparent to those skilled inthe art that the disclosed invention may be modified in numerous waysand may assume many embodiments other than those specifically describedabove. Accordingly, it is intended by the appended claims to cover allmodifications of the invention that fall within the true spirit andscope of the invention.

What is claimed is:
 1. A disk drive comprising: driving means forrotating a disk; a head for reading and/or writing information from/onthe disk; a head moving mechanism for moving the head in a radialdirection of the disk; a DC motor for driving the head moving mechanism;voltage generating means for generating first and second voltages, thefirst voltage having an amplitude high enough to rotate the DC motor tosuch a degree as to get the head moved by the head moving mechanism, thesecond voltage having the same polarity as the first voltage and havingsuch amplitude as to prevent the DC motor from rotating either in abackward direction due to a cogging torque or in a forward direction;and control means for controlling the voltage generating means in such amanner that the voltage generating means applies the second voltage tothe DC motor after having applied the first voltage to the DC motor. 2.The disk drive of claim 1, wherein the control means gets a pulsevoltage applied as the first voltage to the DC motor, a pulse width T ofthe pulse voltage satisfying t≦T≦5 t, where t is an electrical timeconstant of the DC motor.
 3. The disk drive of claim 1, wherein thefirst voltage is three times or more as high as a minimum voltage thatneeds to be applied to the DC motor for the head moving mechanism toovercome a load applied to the head and to move the head.
 4. The diskdrive of claim 1, wherein the control means controls the voltagegenerating means in such a manner that the voltage generating meansalternately applies the first and second voltages to the DC motor. 5.The disk drive of claim 1, wherein the voltage generating meanscomprises a pulse width modulator and generates the first and secondvoltages as effective pulse voltages to be output from the pulse widthmodulator.